The present invention relates to the magnet arts. It finds particular application in conjunction with magnetic resonance imaging apparatus having self-shielded gradient coil assemblies and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with magnetic resonance spectroscopy systems and other applications in which a polarizing magnetic field or a magnetic field with a predetermined contour are advantageous.
Magnetic resonance imagers commonly include a bore having a diameter of 60 cm or more for receiving the body of an examined patient. The bore is surrounded by a series of annular superconducting magnets for creating a substantially uniform magnetic field longitudinally through the patient receiving bore. The longer the magnet, the more uniform the primary magnetic field within the patient receiving bore tends to be (uniform over a sphere). Typically, the length of the bore is about twice the diameter.
Such "long" bore magnets are designed such that there is substantially no measurable degradation from harmonics below twelfth order, commonly denoted Z12. The lower order harmonics which occur as a result of construction inaccuracies are typically shimmed by placing pieces of iron in the bore.
A single magnetic field sensor is moved manually to different locations including positions on 12 axial planes. In each plane the detector is rotated in 12 equal angular increments and the magnetic field is measured. In this manner, 12 measurements are made in each of 12 parallel planes on the surface of a spherical volume of interest centered at the isocenter of the magnet.
Shim trays are removably mounted at a number of equiangular locations around the magnetic bore, e.g., 32 to 36. The shim trays each extend the length of the bore and have 12-14 pockets for receiving plates or shims of iron or steel. The shim trays are removed from the bore and the iron or steel shims are mounted in the pockets. The shim trays are replaced and the magnetic field is measured again to assure that the measured harmonics Z1.fwdarw.Z6 have been minimized. Typically, higher order harmonics require significantly more steel to effect a correction than do lower order harmonics.
One of the disadvantages of long bore magnets is that the region of interest is often inaccessible to medical personnel. If a procedure is to be performed based on the image, the patient must be removed from the bore before the procedure can be performed. Moving the patient increases the risk of misregistration problems between the image and the patient. Other disadvantages of the long bore magnets is that they tend to be user-unfriendly and claustrophobic, larger magnets are more expensive than smaller magnets, and the like.
One way to improve access to the patient is to shorten the length of the magnet and the patient receiving bore. If the magnet and the bore were shortened to about 1 meter or roughly the diameter of the patient bore, patient access would be much improved. Although the size of the uniform magnetic field compresses from a sphere toward a more disk-like shape, the area of substantial uniformity is still sufficient for a series of 10-20 contiguous slice images.
However, shortening the bore of the magnet has its difficulties. First, the magnetic field tends to become non-uniform with the presence of higher order harmonics Z10, Z12, and the like. Second, the inventors herein have measured significant higher order harmonic distortions in such short bore magnets. The Z12 harmonic is found to be relatively strong. When monitoring harmonics with 12 probes, the 12 probes fall midway between Z12 harmonics. Because the 12 probes are measuring magnetic fields at the midpoint of the Z12 harmonics, even significant Z12 harmonics disappear. The inventors herein have found that higher order harmonics require more steel which is positioned with higher positional accuracy.
In accordance with the present invention, a magnetic resonance magnet is provided which is shimmed for improved uniformity through at least the Z12 harmonic.